In the printing industry, a desired image is repeatedly printed on a continuous web or substrate such as paper. In a typical printing process, the continuous web is slit (in the longitudinal direction which is the direction of web movement) to produce a plurality of continuous ribbons. The ribbons are aligned one on top of the other, folded longitudinally, and then cut laterally to produce a plurality of multi-page, approximately page-length segments, each of which is termed a "signature ". As used herein, the term signature also encompasses a single printed sheet that has or has not been folded. Because more than one different signature can be printed at one time, it is often desirable to separate the different signatures by transporting successive signatures in different directions or paths.
One way to accomplish the sorting of a single stream of signatures is to use a diverter mechanism including a stationary diverter wedge to divert successive signatures to one of two paths. Examples of such diverter mechanisms are described in U.S. Pat. Nos. 4,373,713 and 4,729,282.
Another way to accomplish the sorting of a single stream of signatures into two or more streams is with the use of rotating buckets (also known as fans, fan wheels, or paddle fans). One known configuration for sorting signatures includes two sets of rotating buckets, wherein the two sets have outer diameters which overlap. This arrangement is disclosed in U.S. Pat. No. 5,112,033. As described therein, each set of buckets includes several identical buckets arranged at a spaced distance from one another along a common axis. Each bucket has multiple blades which define pockets or slots for receiving signatures. Each blade includes a recess so that the blades do not collide when the two sets of buckets rotate. Signature placement alternates from the slots of the right set of buckets to the slots of the left set of buckets to thereby sort the single stream of signatures into two streams.
The use of recesses in the blades as described in U.S. Pat. No. 5,112,033 has certain disadvantages. For example, a recess in the blade presents an obstacle for a signature and may interfere with the smooth entry or exit of the signature into or out of the slot. The process of removing a signature from a slot is also referred to as "stripping " the signature.
Additionally, in one embodiment described in the above-referenced patent, the recess in the blade has a cover that acts like a spring. The cover can be depressed, and when the force is removed, the cover will bounce back. This cover gets depressed by the blade of the opposite bucket once per signature that is processed in the associated slot. Thus, the cover can wear out, break, or jam in the open or closed position. Also, the dust created from the cutting process can cause problems with jamming of the cover.
It is desirable to increase the operating speed of a printing press as much as possible in order to increase the printed product output. However, as the rotational speed of the buckets is increased, it is more difficult to ensure the reliable operation of the buckets while ensuring the quality of the signatures. For example, signature quality problems that can occur at higher press speeds include ink offset, dog-eared edges, and defects to both the leading and trailing edges of the signatures. These and other defects can lead to paper jams in the folder buckets, resulting in press downtime and expense.